The present invention relates to a receiver and, more particularly, to a receiver for demodulating and decoding a received signal by digitizing it with accuracy high enough for demodulation without changing the characteristic of a transmission path.
It has been customary with a receiver to amplify a received signal coming in through an input terminal by an amplifier, digitize the amplified signal by an analog-to-digital converter (ADC), store the resulting digital value in a memory, and demodulate and decode the received signal based on the digital value stored in the memory. So long as modulation is implemented by frequency modulation or phase modulation and the signal is free from the influence of fading, it is possible to reduce the number of quantizing bits of the ADC by implementing the amplifier as an automatic gain control (AGC) circuit, i.e., by optimizing the input level of the signal to the ADC. Alternatively, a reference input voltage for the ADC and, therefore, a quantizing level may be changed stepwise in accordance with the level of the input signal in order to broaden a dynamic range, as proposed in Japanese Patent Laid-Open Publication Nos. 63-250918 and 61-142823.
Assume that the signal input to the receiver suffers from fading and is corrected by an equalizer, that a QAM (Quadrature Amplitude Modulation) system is used and the position of a signal point on rectangular coordinates is determined by use of a pilot signal which will have a known phase at a known time, or that the QAM system is used and the position of a signal point on rectangular coordinates is determined after fading correction. The prerequisite with any of such schemes is that the characteristic of a transmission path be not changed. Using an AGC circuit or changing the reference input voltage is not practicable since it would change the characteristic of a transmission path. When the receiver receives a signal in the form of bursts, an input signal strength may be estimated beforehand in order to provide the amplifier with a matching amplification factor or to set up a matching reference input voltage; the amplification ratio or the reference input signal will not be changed during the reception of bursts. However, the estimation is impossible in an environment suffering from fading. The only way left is, therefore, using an ADC having a number of bits great enough to cover a broad dynamic range.